Freeze drying works because ice can pass directly to vapour below the triple point of water. The cycle is a controlled trade between three coupled processes: heat into the product, vapour out of the product, and water captured at the condenser. Understanding the driving forces makes the difference between a robust cycle and one that is fragile or inefficient.
The phase diagram and the triple point
At pressures below about 611 Pa (the triple point of water), liquid water is not stable: ice sublimes directly to vapour when heat is supplied. A freeze dryer keeps the chamber below this pressure while the product is frozen, and uses the condenser — held colder than the product — as the sink that keeps water vapour moving out of the product.
Sublimation is driven by a pressure difference
Water leaves the product because the vapour pressure at the ice surface exceeds the pressure in the chamber. The condenser must be cold enough that its equilibrium vapour pressure stays below the chamber pressure, otherwise vapour has nowhere to go and drying stalls. This is why condenser temperature, not just shelf temperature, is a process variable.
Heat must reach the ice, not overheat the dry layer
Heat reaches the frozen interface by conduction through the shelf and container, by conduction through the low-pressure gas, and by radiation from surrounding surfaces. As drying progresses the dry, porous layer grows, insulating the remaining ice. The shelf temperature is raised carefully to keep the product below its collapse or melt limit while still pushing heat through the growing resistance.
Process design. These fundamentals are independent of a specific machine model. The relevant machine evidence is shelf-temperature uniformity, condenser capacity, vacuum control and how well the cycle holds the product within its limits.